Extended Migration Email Coexistence

Email coexistence is used when migrating mailboxes over a period of time for the same email domain, where mail needs to be delivered to two or more different messaging systems.

Set up email coexistence by creating mail forwards for mailboxes that will be migrated. Email is delivered to a primary Source messaging system in which mailboxes that have been moved to a different system will have their mail forwarded.

Proper email delivery flow when coexistence is in place looks like the this:

  1. Email is delivered to the original messaging system to user@example.com.
  2. System detects mailbox has a forward in place.
  3. Email is forwarded to the coexistence email address user@coexist.example.com.
  4. Email arrives at the new messaging system.
  5. Migration is initiated.
  6. Messages from the old system are migrated to new system.

When setting up mail forwards, be sure that the option to have messages stored locally before forwarding has not been selected.  This may result in duplicate messages, if a migration is performed after a forward is put in place. The reason for this is that we will not match up messages created by a forward. We only prevent duplication of messages that were created by us.

An improper email delivery flow when coexistence is in place and migration looks like this:

  1. Email is delivered to original messaging system to user@example.com.
  2. System detects mailbox has a forward in place.
  3. A local copy is saved to the original messaging system.
  4. Email is forwarded to the coexistence email address user@coexist.example.com.​
  5. Email arrives at the new messaging system.
  6. ​Migration is initiated.
  7. Messages are migrated, including forwarded messages as they are not matched up.
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